1. Field of the Invention
The invention relates generally to drivers for fasteners of the types disclosed in U.S. Pat. Nos. 2,677,985, 4,590,825 and 4,670,927, which have arcuately slotted heads and have come to be known as "high torque" fasteners. Accordingly, drivers for such fasteners have come to be known as "high torque" drivers.
2. Description of the Prior Art
High torque fasteners, such as those disclosed in the aforementioned U.S. patents, typically have a fastener head provided with a generally bow tie-shaped driving slot defined by an arcuately concave bottom wall separating slightly undercut sidewalls. This driving slot is adapted for reception of and engagement by a driving tool blade having an arcuately convex end surface generally matching the curvature of the slot bottom wall. Upon-rotation of the driving tool, the undercut geometry of the slot sidewalls cooperates with the driving tool blade to accommodate relatively high torque transfer between the driving tool and the fastener head.
One difficulty experienced with such prior art high torque driving tools and fasteners is a tendency of the driving tool blade to cam out of the driving slot of the fastener head. Since the slot is arcuately concave, its depth, and, therefore the depth of engagement between the driving tool blade and the slot, reduces gradually to zero at the ends of the slot. Thus, in high torque applications, the tool has a tendency to cam out of the slot, beginning at the ends of the slot. This tendency can be aggravated by the buildup of dirt or other debris in the driving slot. Such debris prevents the driving tool blade from bottoming in the slot, thereby reducing even further the engagement depth.
In one form of prior art high torque driving tool, the tool blade has a conical tip which projects axially from the end surface for engagement in a mating conical recess in the driving slot of the fastener to facilitate centering the blade in the slot. This prior art centering tip extends axially inboard of the arcuate end surface of the blade and has a base diameter which is substantially greater than the thickness of the blade at the end surface. This configuration is difficult to manufacture, requiring the use of expensive investment casting techniques.